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[英]Is it possible to implement reverseOrder() method using Comparator<T>?
[英]Java Comparator using .reverseOrder() but with an inner class
我正在创建一个简单的程序来了解Java Comparator类。 我已经按顺序对一个Arraylist
了排序,但现在我想按降序对列表进行排序,但是我在调用.reverseOrder()
方法时遇到问题,因为我使用了一个实现Comparator<Song>
的内部类(歌曲是一首歌)容纳吸气剂和制定者方法的类)。
这是我的SongSort
课程,其中包含分类过程等。
import java.util.*;
import java.io.*;
public class SongSort
{
ArrayList<Song> songList = new ArrayList<Song>();
public void main(String[] args)
{
new SongSort().go();
}
class ArtistCompare implements Comparator<Song>
{
public int compare(Song one, Song two)
{
return one.getRating().compareTo(two.getRating());
}
}
public void go()
{
getSongs();
System.out.println(songList);
//Collections.sort(songList);
System.out.println(songList);
ArtistCompare artistCompare = new ArtistCompare();
Collections.sort(songList, artistCompare);
System.out.println(songList);
}
public void getSongs()
{
try{
File file = new File("SongListMore.txt");
BufferedReader reader = new BufferedReader(new FileReader(file));
String line = null;
while((line = reader.readLine()) != null)
{
addSong(line);
}
}
catch(Exception ex)
{
ex.printStackTrace();
}
}
public void addSong(String lineToParse)
{
String [] tokens = lineToParse.split("/");
Song nextSong = new Song(tokens[0], tokens[1], tokens[2], tokens[3]);
songList.add(nextSong);
}
}
这是我简单的Song
课;
public class Song //implements Comparable<Song>
{
private String title;
private String artist;
private String rating;
private String bpm;
public Song(String t, String a, String r, String b)
{
title = t;
artist = a;
rating = r;
bpm = b;
}
public String getTitle()
{
return title;
}
public String getArtist()
{
return artist;
}
public String getRating()
{
return rating;
}
public String getBpm()
{
return bpm;
}
public String toString()
{
return ("Title : " + title + "," + " Artist : " + artist + " Rating : " + rating);
}
}
任何人都可以帮我弄清楚我将在SongSort
类中调用reverseOrder()
方法,因为它不会编译?
ArtistCompare artistCompare = new ArtistCompare();
Collections.sort(songList, Collections.reverseOrder(artistCompare));
编辑2015年7月
由于这个答案仍然受到一些关注,这里有一个小小的更新:
使用Java SE 8,创建反向比较器变得更加容易:
Comparator<Song> songRatingComparator = Comparator.comparing(Song::getRating);
Collections.sort(songList, songRatingComparator.reversed());
当然,您也可以使用Streams框架:
List<Song> sortedSongList = songList.stream()
.sorted(Comparator.comparing(Song::getRating).reversed())
.collect(Collectors.toList());
实现逆序比较器的一种方法是实现反转比较器结果的Compartor-Delegate(通过改变顺序)。
public class ReverseOrder<T> implements Comparator<T> {
private Comparator<T> delegate;
public ReverseOrder(Comparator<T> delegate){
this.delegate = delegate;
}
public int compare(T a, T b) {
//reverse order of a and b!!!
return this.delegate.compare(b,a);
}
}
所以你唯一需要做的就是使用这个委托。 例如:
Comparator myComparator = new myComparator();
List list = ...;
List reverse = new ArrayList(list);
//acceding
Collections.sort(list, myComparator);
//descending
Collections.sort(list, new ReverseOrder(myComparator));
让我们举一个简单的例子,我们有一个Person有两个字段名称age的类,我们想根据它们的年龄对现有的一组人进行排序,所以让我们假设我们有一个带有构造函数的Person,并将这些人添加到列表中然后排序他们没有方法的集合类:
Person bachiri = new Person (17,"bachiri");
Person taoufiq = new Person (14,"Taoufiq");
Person abderrahman = new Person (15,"abderrahman");
List<Person> persons = new ArrayList<>();
而这就是Agecomparable的阻碍:
class AgeComparator implements Comparator<Person>{
@Override
public int compare(Person person1, Person person2) {
return Integer.compare(person1.getAge(),person2.getAge());
}
}
诀窍是使用-1复用return方法,以便最终结果反转:class AgeComparator实现Comparator {
@Override
public int compare(Person person1, Person person2) {
return -1 * Integer.compare(person1.getAge(),person2.getAge());
}
}
所以现在我们可以得到相反的结果:
Collection.sort (Persons, new AgeComparator());
如果需要使用反转当前顺序的比较器,只需在compare
方法中返回负值 。
public class ComparatorInverse implements Comparator<Object> {
@Override
public int compare(Object lhs, Object rhs) {
return -1;
}
}
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